Re: [PATCH 10/10] mm: page_alloc: consolidate free page accounting

[Baolin Wang]

On 2024/4/9 22:46, Zi Yan wrote:
> On 9 Apr 2024, at 5:31, Baolin Wang wrote:
>
> > On 2024/4/8 22:23, Johannes Weiner wrote:
> > > On Mon, Apr 08, 2024 at 09:38:20AM +0200, Vlastimil Babka wrote:
> > > > On 4/7/24 12:19 PM, Baolin Wang wrote:
> > > > > On 2024/3/21 02:02, Johannes Weiner wrote:
> > > > > >    +	account_freepages(page, zone, 1 << order, migratetype);
> > > > > > +
> > > > > >     	while (order < MAX_PAGE_ORDER) {
> > > > > > -		if (compaction_capture(capc, page, order, migratetype)) {
> > > > > > -			__mod_zone_freepage_state(zone, -(1 << order),
> > > > > > -								migratetype);
> > > > > > +		int buddy_mt = migratetype;
> > > > > > +
> > > > > > +		if (compaction_capture(capc, page, order, migratetype))
> > > > > >     			return;
> > > > > > -		}
> > > > >
> > > > > IIUC, if the released page is captured by compaction, then the
> > > > > statistics for free pages should be correspondingly decreased,
> > > > > otherwise, there will be a slight regression for my thpcompact benchmark.
> > > > >
> > > > > thpcompact Percentage Faults Huge
> > > > >                              k6.9-rc2-base        base + patch10 + 2 fixes	
> > > > > Percentage huge-1        78.18 (   0.00%)       71.92 (  -8.01%)
> > > > > Percentage huge-3        86.70 (   0.00%)       86.07 (  -0.73%)
> > > > > Percentage huge-5        90.26 (   0.00%)       78.02 ( -13.57%)
> > > > > Percentage huge-7        92.34 (   0.00%)       78.67 ( -14.81%)
> > > > > Percentage huge-12       91.18 (   0.00%)       81.04 ( -11.12%)
> > > > > Percentage huge-18       89.00 (   0.00%)       79.57 ( -10.60%)
> > > > > Percentage huge-24       90.52 (   0.00%)       80.07 ( -11.54%)
> > > > > Percentage huge-30       94.44 (   0.00%)       96.28 (   1.95%)
> > > > > Percentage huge-32       93.09 (   0.00%)       99.39 (   6.77%)
> > > > >
> > > > > I add below fix based on your fix 2, then the thpcompact Percentage
> > > > > looks good. How do you think for the fix?
> > > >
> > > > Yeah another well spotted, thanks. "slight regression" is an understatement,
> > > > this affects not just a "statistics" but very important counter
> > > > NR_FREE_PAGES which IIUC would eventually become larger than reality, make
> > > > the watermark checks false positive and result in depleted reserves etc etc.
> > > > Actually wondering why we're not seeing -next failures already (or maybe I
> > > > just haven't noticed).
> > >
> > > Good catch indeed.
> > >
> > > Trying to understand why I didn't notice this during testing, and I
> > > think it's because I had order-10 pageblocks in my config. There is
> > > this in compaction_capture():
> > >
> > > 	if (order < pageblock_order && migratetype == MIGRATE_MOVABLE)
> > > 		return false;
> > >
> > > Most compaction is for order-9 THPs on movable blocks, so I didn't get
> > > much capturing in practice in order for that leak to be noticable.
> >
> > This makes me wonder why not use 'cc->migratetype' for migratetype comparison, so that low-order (like mTHP) compaction can directly get the released pages, which could avoid some compaction scans without mixing the migratetype?
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 2facf844ef84..7a64020f8222 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -622,7 +622,7 @@ compaction_capture(struct capture_control *capc, struct page *page,
> >           * and vice-versa but no more than normal fallback logic which can
> >           * have trouble finding a high-order free page.
> >           */
> > -       if (order < pageblock_order && migratetype == MIGRATE_MOVABLE)
> > +       if (order < pageblock_order && capc->cc->migratetype != migratetype)
> >                  return false;
> >
> >          capc->page = page;
>
> It is worth trying, since at the original patch time mTHP was not present and
> not capturing any MIGRATE_MOVABLE makes sense. But with your change, the capture
> will lose the opportunity of letting an unmovable request use a reclaimable
> pageblock and vice-versa, like the comment says. Please change the comment
> as well and we should monitor potential unmovable and reclaimable regression.

Yes, but I think this case is easy to solve. Anyway let me try to do 
some measurement for mTHP.